The switching of signal light beams between optical fiber bundles carrying data optical I/O cards in communication networks and systems is currently being addressed with the aid of optical microelectric-electromechanical devices (MEMS or MOEMS) carrying tiltable micro-mirrors orientable by electrical signal control to deflect incident light beams along desired paths—switching the output light from input fibers via reflection from the micro-mirrors to predetermined fibers in an output fiber bundle—the so-called optical cross-connect switching mentioned above.
Examples of such MEMS devices are disclosed, for example, in U.S. Pat. Nos. 6,052,498; 6,147,876; 6,150,724; and 6,151,173; and details of such optical switching systems are described, for example, in the following references:                [1] “Flexible, Modular, Contact Fiber Optic Switch”, Slater, et al., Xros Patent Application, WO 00/20899. Apr. 13, 2000;        [2] “Sensing Configuration for Fiber Optic Switch Control System”, Laor, et al., Astarte Fiber Networks, Inc., U.S. Pat. No. 6,097,858, Aug. 1, 2000;        [3] “Optical MEMS for Lightweight Networks”, David Bishop, SC233, SPIE Photonics East, Nov. 8, 2000.        
In general, such free-space optical switches steer the input light from any one of input fibers to any one of the output fibers. The number of input fibers and output fibers can range from tens to thousands. The principal challenges to controlling the light are providing for:                quickly moving one or multiple light beams from one location to another without disturbing the existing connections;        fine adjusting of the light beam so that the beam is precisely placed into the optimal coupling range of the output fiber, so that the optical signal loss due to the coupling can be reduced to minimal—this loss being the major contributor to the total insertion loss of the optical switch;        precisely maintaining the connections despite possible mechanical or thermal disturbances;        establishing multiple connections simultaneously as requested by higher level provisioning software and within certain prescribed ranges;        adapting to the switch device architecture, as by providing the micro-mirror array chips with on-chip actuation and sensing circuitry; and        minimizing the space and computation power requirements for off-chip signal processing and control systems.        
In the implementation of, for example, the optical switches of the above-cited Slater et al reference [1] and other similar prior art systems, electrical signal feedback circuits for establishing precise mirror position are widely used.
A piezoresistive sensing mechanism is generally provided on the torsional mirror support to measure position of the mirror. This sensing signal is fed to an error amplifier with a command input from digital signal processor a (DSP)-controlled digital analog controller (DAC). The error signal is fed through the analog controller to control the actuation or control voltage signal orienting the mirror. Problems and limitations with this electrical feedback approach, however, arise from the facts that the controller is implemented in analog components which are not flexible; the controller is not scalable because each mirror needs its control; and the DSP is also not scalable because it is tied to the controller all of the time.
Resort has therefore been taken, as described in the Bishop reference [3] above, to the use of external optical feedback without using such on-chip electrical feedback signals—i.e. using only port card optical power feedback. Since such involve a very slow loop, however, this poses extremely high requirements on MEMS design, in that the resonant frequency has to be much higher than the disturbance. The optical power feedback from the I/O cards, furthermore, cannot give directional information. This thus requires extra time and computation power to adjust the mirror beam positions. While additional optical sensing schemes can be built into the optical box as in the Bishop proposal, this further increases the complexity of the optical box and adds more variables to control, i.e. the relative positions and alignment of the optical sensing elements in the optical box.
The present invention, on the other hand, through a novel use of intermediate node monitoring and with the use of test and real traffic data path switching, has admirably overcome the above-described and other limitations of such prior art techniques, as will later be more fully explained.